1. Field of the Invention
The present invention relates to a band limiter that is capable of limiting a pass-band of of an output signal of an amplifier so that only a low frequency component of the output signal is passed when the band limiter is in operation, and in particular, related to a band limiter including a temperature compensation circuit for compensating the high frequency characteristic of the amplifier for temperature changes when the band limiter is not in operation.
2. Description of the Related Art
Band limiters are applied to various apparatuses such as oscilloscopes. For example, in an oscilloscope, a band limiter is used for narrowing a pass-band of a signal in order to eliminate noises in a waveform to be observed. Such a band limiter is connected to an output terminal of a vertical amplifier whose frequency characteristic is as shown in FIG. 1A, so that the frequency characteristic of the output of the vertical amplifier is changed to that as shown in FIG. 1B.
When a waveform including step-like transitions is observed by the oscilloscope, however, the band limiter will round the waveform as shown in FIG. 2A. Accordingly, to observe such waveforms, the band limiter is generally disconnected from the output of the vertical amplifier.
The vertical amplifier disconnected from the band limiter, however, changes its high frequency characteristics in accordance with ambient temperature in the oscilloscope. For example, when the temperature increases, attenuation in the high frequency band generally increases as indicated by a dashed-and-dotted line of FIG. 1C, whereas when the temperature decreases, attenuation in the high frequency band decreases as indicated by a broken line in FIG. 1C. Consequently, a step-like transition of a waveform will round as shown in FIG. 2B when the temperature increases, whereas it will overshoot or undershoot as shown in FIG. 2C when the temperature decreases.
To prevent these rounding, overshoot and undershoot, a conventional oscilloscope incorporates a compensation circuit for canceling changes in high frequency characteristics according to temperature changes, in addition to the band limiter. This compensation circuit enables a waveform to be correctly displayed on a CRT as shown in FIG. 2D.
The compensation circuit comprises a thermo-sensitive device, a variable resistor and a variable capacitor which are controlled by an output signal of the thermo-sensitive device. Typically, a thermistor is used as the thermo-sensitive device, a field-effect transistor is employed as the variable resistor, and a small capacitance varicap as used as the variable capacitor.
The conventional compensation circuit, however, has the following problems.
First, careful circuit design is required because of strict circuit design conditions imposed on such a high frequency circuit. Second, the direct current voltage applied to the compensation circuit must be carefully handled so that it does no harm to the output signal of the vertical amplifier.